Conventional earth boring drill bits of the roller cone or rotary conical cutter type have discrete chisel or wedge-shaped teeth with their crests extending longitudinally of their respective cone and in planes approximately radially of the respective axis of rotation thereof or transversely of its respective direction of rotation. Usually, these teeth have a taper in the range of 42.degree. to 46.degree. in order to prevent excessive tooth breakage, and this tooth angle is maintained regardless of the density (soft, medium, hard) of the formation to be drilled as well as of the length and spacing of said teeth. In addition to the circumferential spacing thereof, the teeth are disposed in spaced circumferential or concentric rows with the rows of each cone being offset relative to the rows of adjacent cones. In most instances, the length of and the distance between adjacent circumferential teeth is determined by the density of the earth formation to be drilled, which is generally classified as soft, medium or hard and which varies according to the strata of the earth, with such teeth length and distance therebetween decreasing with increasing density or hardness and with the shortened teeth being as close together as possible. It is noted that formations of varying density or hardness are disposed frequently in adjacent strata of relative thinness.
The aforesaid approximate 42.degree. to 46.degree. taper results in thickened crests or cutting edges, which thickness increases in proportion to the extent of wear of the teeth and is amplified when said teeth are relatively long, thereby requiring the application of additional weight in order to penetrate the formation. Obviously, the applied weight must not exceed the bearing capacity of the roller cones or the total weight of the collars of a string of drill pipe. Teeth of this type are prone to breakage when a denser or harder formation is encountered, and attempts to reduce this tendency have included increasing the taper and the thickness or lesser transverse dimension of the teeth (tooth width transverse to taper) and/or shortening the length of said teeth; however, this manner of strengthening the teeth, again results in thickened crests or bread cutting surfaces upon wear of the teeth so as to resist penetration of the formation and necessitate the application of additional weight on the drill bit in order to obtain a satisfactory penetration rate.
Drill bit cones having rows of closely spaced teeth for use in medium to medium hard formations usually create a pattern in the formation known as "gearing", the teeth tending to contact the formation in depressions formed by said teeth during a previous revolution of the cone. Due to this "gearing" action, ridges are created between adjacent depressions which strengthen the formation and resist fracture thereof by the teeth.
In a drill bit cone having tooth crests extending longitudinally thereof and in planes extending approximately radially of the axis of rotation thereof, the depth of penetration of each tooth into the formation is restricted by the engagement of its adjacent teeth in each row of teeth with said formation whereby the amount of penetration is dependent upon the distance between said teeth. As shown in FIGS. 5-8, each center tooth of any group of three adjacent teeth in any row penetrates the formation until the following or trailing tooth contacts the formation. Since the lead or preceding tooth remains in engagement at this point, penetration ceases because all three of the teeth attempt to enter the formation at different angles whereby additional penetration is impossible. This action is repeated by each group of three teeth and determines the maximum penetration by a drill bit during each revolution. As each cone rotates on its axis, the following tooth of each of the aforesaid groups presents a flat surface to the formation (FIG. 5), and this tooth cannot penetrate until the cone revolves sufficiently to position said tooth directly under said cone so that its cutting edge or crest fractures and forces the formation to either side of said tooth.
If the teeth are widely spaced circumferentially to permit deeper penetration by each tooth in turn, the following or trailing tooth may break as the load or weight of the drill string is applied to its flat side. If it does not break, this tooth must lift the load until it is directly beneath the cone so as to penetrate the formation. Another disadvantage is that in soft or gummy formations, such as gumbo or shale, cuttings pack in the recesses between the flat surfaces of adjoining teeth in each row and the shell of the cone. This mass of cuttings is dehydrated by engagement with the formation and the hydrostatic pressure of the fluid column in the well hole, this condition being commonly known as and referred to as bit balling (a balled up drill bit). It is noted that the mass of cuttings between adjacent teeth becomes firm and dense and may extend to the crest of said teeth, whereby it supports the weight applied to a drill bit and prevents penetration into the formation by said teeth.
Another problem frequently encountered is known as "tracking" which is caused by a circumferential row of teeth sliding into a depression caused by an offset row of teeth on one of the other cones. This action leaves circumferential ridges of uncut formation which create wear on the ends of the radial teeth crests and stop penetration since the height of the ridges exceeds the remaining tooth length or height and the cone shell or exterior rests on said formation ridges. Also, the "tracking" action causes the guage surfaces of a drill bit to abrade the wall of the well hole and drill a bore which is of greater diameter than said bit. Under these conditions penetration of the bit ceases and it must be replaced by another type.
In order to increase the rate of penetration in formations, such as shale, the roller cones of a conventional drill bit have the inner ends of their axes of rotation offset relative to the axis of rotation of said bit whereby said cones bear outwardly so as to provide radial or transverse movement for more efficiently fracturing the formation, particularly the annular ridges formed therein during previous revolutions. Unfortunately, the rotation of the drill bit causes the radial teeth of each cone to slide inwardly toward the rotational axis of said bit; and the resultant abrasion of said teeth causes more rapid wear thereof and increases the radial load or thrust exerted on the bearings of the cones so as to appreciably shorten the life of said bearings.